1. Field of the Invention
The present invention relates to a multilayer optical recording medium having three or more recording layers with less interlayer crosstalk generated.
2. Description of the Related Art
FIG. 1A shows a cross-sectional structure of a conventionally known multilayer optical disk among optical recording media, and schematically shows the principle for selectively recording and reproducing information of each recording layer. In the example of FIG. 1A, the recording medium includes a total of six layers having information recorded thereon (hereafter referred to as recording layers), and these six layers are referred respectively to as a first recording layer 101, a second recording layer 102, a third recording layer 103, a fourth recording layer 104, a fifth recording layer 105, and a sixth recording layer 106 in the order starting from a side opposite to the side where the irradiation light is incident on the optical disk, i.e., from the bottom side in FIG. 1A. For accessing recorded information, for example, on the third recording layer 103 by using this six-layer medium, the position of an objective lens is controlled in an optical recording and reproducing apparatus so that an optical spot 107 is positioned on the third recording layer 103. At that time, focused light 108 while being narrowed down by the objective lens transmits through the sixth recording layer 106, the fifth recording layer 105, and the fourth recording layer 104 that are semitransparent. On the sixth, fifth and fourth recording layers, the beam diameter of the focused light 108 is sufficiently larger than the diameter of the optical spot 107 on the third recording layer 103. Therefore, recording information on the sixth, fifth, and fourth recording layers 106, 105, and 104 that are semitransparent cannot be resolved and reproduced. In addition, the light intensity per unit area is relatively small since the beam diameter is large on the sixth, fifth, and fourth recording layers 106, 105, and 104 that are semitransparent. Accordingly, there is no risk of destroying information on the sixth, fifth, and fourth recording layers 106, 105, and 104 at the time of recording information. In this way, information recording and reproducing on the third recording layer located far from the incident side of the irradiation light are achieved without being influenced by the sixth, fifth, and fourth recording layers. Information recording and reproducing on other recording layers are conducted in the same way by controlling the position of the objective lens. Conditions for thus conducting recording and reproducing information on an optical recording medium having multiple recording layers without any influence upon other layers are described in detail in Japanese Patent Application Publication No. Hei 5-101398.
In reproduction of the multilayer optical disc, problems arise due to the effect of the light attenuation in recording layers located in front of a target recording layer seen from the irradiation light incident side (hereinafter, “in front of” means the same as here), and also of an influence caused by multiple reflection of light in recording layers located in front of the target recording layer for the recording and reproducing information. How the multiple-reflected light adversely affects information recording and reproducing will be described with reference to FIG. 2. Hereinbelow, the target recording layer of recording and reproducing information is represented as an n-th layer. Incident light 108 formed of focused light is emitted so as to form an optical spot 107 on the n-th layer as shown in FIG. 2. At this time, light reflected by the (n+1)th layer located right in front of the target recording layer becomes unnecessary light 402. After arriving at the back surface of the (n+2)th layer and being reflected by the back surface of the (n+2)th layer, the unnecessary light 402 might be reflected by the (n+1)th layer again, follow nearly the same path of light reflected by the n-th layer, and be detected as if it is light reflected by the n-th layer. In this case, large interlayer crosstalk is generated. In the present specification, “crosstalk” means “interlayer crosstalk.” It is pointed out in Japanese Journal of Applied Physics, Vol. 43, No. 7B, 2004, pp. 4983-4986 and Ushiyama et. al., Tech. Digest of ODS2006, WDPDP3 that detection of such unnecessary light poses a big problem.
For this reason, as the method of reducing the interlayer crosstalk, Japanese Journal of Applied Physics, Vol. 43, No. 7B, 2004, pp. 4983-4986 discloses an example of a multilayer medium having its interlayer distance changed, and Japanese Patent Application Publication Nos. 2006-48811 and 2007-220150 disclose examples of a multilayer medium having its reflectivity controlled by devising layers that are stacked on a recording layer.